1. Field of the Invention
This invention relates generally to the evaluation of a formation penetrated by a wellbore. More particularly, this invention relates to downhole sampling tools capable of collecting samples of fluid from a subterranean formation.
2. Description of the Related Art
The desirability of taking downhole formation fluid samples for chemical and physical analysis has long been recognized by oil companies, and such sampling has been performed by the assignee of the present invention, Schlumberger, for many years. Samples of formation fluid, also known as reservoir fluid, are typically collected as early as possible in the life of a reservoir for analysis at the surface and, more particularly, in specialized laboratories. The information that such analysis provides is vital in the planning and development of hydrocarbon reservoirs, as well as in the assessment of a reservoir's capacity and performance.
The process of wellbore sampling involves the lowering of a downhole sampling tool, such as the MDT™ wireline formation testing tool, owned and provided by Schlumberger, into the wellbore to collect a sample (or multiple samples) of formation fluid by engagement between a probe member of the sampling tool and the wall of the wellbore. The sampling tool creates a pressure differential across such engagement to induce formation fluid flow into one or more sample chambers within the sampling tool. This and similar processes are described in U.S. Pat. Nos. 4,860,581; 4,936,139 (both assigned to Schlumberger); U.S. Pat. Nos. 5,303,775; 5,377,755 (both assigned to Western Atlas); and U.S. Pat. No. 5,934,374 (assigned to Halliburton).
Various challenges may arise in the process of obtaining samples of fluid from subsurface formations. Again with reference to the petroleum-related industries, for example, the earth around the borehole from which fluid samples are sought typically contains contaminates, such as filtrate from the mud utilized in drilling the borehole. This material often contaminates the clean or “virgin” fluid contained in the subterranean formation as it is removed from the earth, resulting in fluid that is generally unacceptable for hydrocarbon fluid sampling and/or evaluation. As fluid is drawn into the downhole tool, contaminants from the drilling process and/or surrounding wellbore sometimes enter the tool with fluid from the surrounding formation.
To conduct valid fluid analysis of the formation, the fluid sampled preferably possesses sufficient purity to adequately represent the fluid contained in the formation (ie. “virgin” fluid). In other words, the fluid preferably has a minimal amount of contamination to be sufficiently or acceptably representative of a given formation for valid hydrocarbon sampling and/or evaluation. Because fluid is sampled through the borehole, mudcake, cement and/or other layers, it is difficult to avoid contamination of the fluid sample as it flows from the formation and into a downhole tool during sampling. A challenge thus lies in obtaining samples of clean fluid with little or no contamination.
Various methods and devices have been proposed for obtaining subsurface fluids for sampling and evaluation. For example, U.S. Pat. No. 6,230,557 to Ciglenec et al., U.S. Pat. No. 6,223,822 to Jones, U.S. Pat. No. 4,416,152 to Wilson, U.S. Pat. No. 3,611,799 to Davis and International Pat. App. Pub. No. WO 96/30628 have developed certain probes and related techniques to improve sampling. Other techniques have been developed to separate virgin fluids during sampling. For example, U.S. Pat. Nos. 6,301,959 to Hrametz et al. and discloses a sampling probe with two hydraulic lines to recover formation fluids from two zones in the borehole. Borehole fluids are drawn into a guard zone separate from fluids drawn into a probe zone. U.S. patent application Ser. No. 10/184833, assigned to the assignee of the present invention, provides additional techniques for obtaining clean fluid as the formation fluid is drawn into the downhole tool. Despite such advances in sampling, there remains a need to develop techniques for fluid sampling that optimize the quality of the sample.
In considering existing technology for the collection of subsurface fluids for sampling and evaluation, there remains a need for apparatuses and methods capable of removing contaminated fluid and/or obtaining acceptable formation fluid. It is, therefore, desirable to provide techniques for removing contamination from the downhole tool so that cleaner fluid samples may be captured. It is also desirable to have a system that optimizes the pump utilization and the contamination level of the sample, while reducing the chances of the tool getting stuck. The present invention is directed to a method and apparatus that may solve or at least reduce, some or all of the problems described above.